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Linksys SPA9x2 Phone Administration Guide 38
Ensuring Voice Quality
Setting System Features
Bandwidth Requirements
Depending on how you have your IP phones configured, each call requires 55 to 110 kbps in
each direction. Therefore, using G.729 as the voice codec setting, and with an average business-
grade broadband Internet connection supporting 1.5 Mbps downstream and 384 kbps
upstream, a total of seven (7) simultaneous conversations can be reliably supported with
adequate bandwidth available for file downloads.
Linksys recommends using the SPA9x2 with QoS-capable networking equipment that can
prioritize the VoIP application traffic. QoS features are available on many Linksys data
networking switches and routers. A QoS-enabled router prioritizes the packets going upstream
to the ISP.
The following table approximates the bandwidth budget for each side of the conversation (in
each direction) using different codecs and number of calls. This table is based on the following
assumptions:
• Bandwidth calculated with no silence suppression
• 20 millisecond of payload per RTP packet
G.726 Low complexity. Supports compressed 16, 24, 32, and 40 kbps
digitized voice transmission at one through ten 10 ms voice
frames per packet. When no static payload value is assigned per
RFC 1890, SPA9x2 phones can support dynamic payloads for
G.726
4.1 (32 kbps)
G.729 and
G.729A
G.729A low-medium complexity. G.729 medium complexity.
G.729A requires about half the processing power of G.729.
The G.729 and G.729A bit streams are compatible and
interoperable, but not identical.
4
G.723.1 High complexity. SPA9x2 phones support the use of ITU G.723.1
audio codec at 6.4 kbps. Up to two channels of G.723.1 can be
used simultaneously. For example, Line 1 and Line 2 can be using
G.723.1 simultaneously, or Line 1 or Line 2 can initiate a three-
way conference with both call legs using G.723.1.
3.8
Codec
Est. Bandwidth
Budget
2 Calls 4 Calls 6 Calls 8 Calls
G.711 110 kbps 220 kbps 440 kbps 660 kbps 880 kbps
G.726-40 87 kbps 174 kbps 348 kbps 522 kbps 696 kbps
G.726-32 79 kbps 158 kbps 316 kbps 474 kbps 632 kbps
G.726-24 71 kbps 142 kbps 284 kbps 426 kbps 568 kbps
Codec (Voice
Compression
Algorithm)
Complexity and Description MOS Score